This invention relates to a stereophonic reproduction system having right and left channels for reproducing sound sources.
Stereophonic reproduction aims at high-fidelity reproduction of an original sound field, and provides right and left speakers arranged symmetrically with respect to a listening position for realizing a feeling of spread and desired sound image localization.
However, there are occasions on which a listener cannot occupy an equidistant position to the right and left speakers, which is the basic condition for appreciating stereophonic reproduction.
Particularly in an automobile, compared with an ordinary listening room, it is impossible to listen at an equal distance to the right and left speakers because of the limitations to speaker mounting positions and the seat positions. Where, as in this case, the listener stays at a biased position relative to the speaker arrangement, there occurs a phase difference due to a time lag between sound signals resulting from the spatial propagation velocity of sound waves from the speakers to the listening position. Consequently, a state close to antiphase takes place between the two ears of the listener, wherein the sound signals counteract each other at a particular frequency to deteriorate amplitude characteristics and provide a markedly anitphasal, unnatural sound image localization.
To cope with the above inconvenience, correction is made to sound image localization in a limited sound field such as an automobile interior.
Japanese Utility Model Publication Kokai No. 63-49900, for example, discloses a system including a phase shifter in at least one of the signal paths for the right and left channels for varying the phase of a signal in a selected frequency band. Phase compensation is electrically made for the propagation delay of sound signals due to spatial propagation distances from the right and left speakers to a listening position, thereby to compensate for the relative phase difference at the listening position between the sounds from the two speakers.
The principle of the above known sound image localization correcting device will be described hereinafter with reference to the accompanying drawings.
As shown in FIG. 8, a phase shifter having phase characteristics Ph(f), for simplicity of explanation, is provided on one of the signal paths for correcting spatial propagation delays at various frequencies in a frequency band as used. The other signal path allows through-pass.
Assume that, in FIG. 8, the listener is seated on the driver's seat 5R, and that the phase shifter has characteristic Ph(f) which is the function of frequency f, for correcting, over the frequency band, the phase difference of spatial sound propagation time from a left speaker 4L to the driver's seat 5R relative to the spatial sound propagation time from a right speaker 4R to the driver's seat 5R.
Where no correction is made, the phase difference due to the above-mentioned spatial propagation delays will disturb spatial composite frequency characteristics and sound image localization. Especially where the relative phase difference between the right and left speakers is 180 degrees at a particular frequency, the sound signals from the two speakers are canceled for that frequency, thereby disturbing the frequency characteristics.
Where the phase shift 2 having the phase characteristic Ph(f) is mounted on the right signal path, the right speaker 4R reproduces a signal with a delay of Ph(f) electrically set in advance by the phase shift 2, for transmission to the driver's seat. The left speaker 4L reproduces a signal not electrically delayed, which is subjected to the spatial sound propagation delay Ph(f) in the sound field before reaching the driver's seat. As a result, there occurs zero phase difference at the driver's seat between the sounds reproduced from the right and left speakers.
For the passenger seat 5L next to the driver's seat, on the other hand, the left speaker 4L reproduces a signal not electrically delayed. The right speaker 4R reproduces a signal with the delay of Ph(f) electrically set in advance by the phase shift 2, for transmission to the passenger seat. Besides, this signal is subjected to the spatial sound propagation delay Ph(f) in the sound field before reaching the passenger seat. As a result, there occurs a phase difference 2Ph(f) at the passenger seat 5L between the sounds reproduced from the right and left speakers.
The correction value Ph(f) has a maximum value 180 degrees based on what is known as the phase cyclicity. Thus, the relative phase difference at the passenger seat 5L is 360 degrees and, because of the phase cyclicity, the problem of anitphasal, unnatural sound image localization noted hereinbefore.
The above is an explanation of the principle of correction made to the sound image localization at a biased listening position according to the prior art.
However, with the known correcting method as described above, which corrects the phase characteristics by means of the phase shifter having phase characteristics Ph(f) and provided for one of the right and left channels, has the disadvantage of disturbing the harmonic structure at a listening position of spatial composite sound signals of various musical instruments, and greatly deteriorating tone quality.
That is, the spatial sound propagation delay Ph(f) is determined by relation between a difference between distances from the right and left speakers to the listening position, and wavelengths of sound signals in the frequency band used. Its phase characteristics are almost zero in the low frequency range (20 to 100 Hz), zero to 180 degrees in the medium frequency range (100 Hz to 1 KHz) and almost zero in the high frequency range (1 to 20 KHz).
Since a phase shifter having such phase characteristics is mounted on a signal path, those instruments having basic sounds in the low frequency range, for example, have their harmonic components distributed to the medium and high frequency ranges. In the medium frequency range in particular, the phases are shifted to a maximum of 180 degrees. Since the phases of the harmonic components twice or three times the basic sounds are shifted to the maximum of 180 degrees, there occur changes in the structure of sound spectrum of the sound signals, or the formants characteristic of instruments, reproduced from the speakers, thereby deteriorating the tone qualities of various instruments.
Many musical instruments are known to have basic sound components in the low frequency range not exceeding 100 Hz. FIGS. 3(a) and (b) show the sound spectral distributions of the violin and the bass for reference, FIG. 3(a) showing the sound spectral distribution of the former and FIG. 3(b) that of the latter. It will be seen that, in the case of the bass, for example, the basic sound is at 300 Hz and most of the tertiary and further harmonic components are distributed in the medium frequency range (100 Hz to 1 KHz).